1. Field of the Invention
This invention relates generally to assays for determining the presence or concentration of an analyte in a fluid sample and, more specifically, to improvements in the architecture of test strip devices used in such assays. The invention relates especially to such devices for use in the determination of analytes in blood.
2. Description of the Prior Art
Teststrips are not new. Their use in determining the presence or concentration of an analyte, such as glucose, in a sample, such as blood or urine, is well accepted. Generally, teststrips operate on the principle that the analyte causes a detectable change which can be used to determine the presence or concentration of the analyte.
Some such teststrips indicate the presence or amount of an analyte by a degree of color change in a reagent. Some color change teststrips are intended to be read visually while others are intended to be interrogated by an electronic meter, such as a reflectance photometer which looks for shifts in color intensity or reflectance at a specific wavelength.
In other teststrips the presence or amount of an analyte is indicated by a change in the electrical condition of a reagent. Such teststrips are commonly known as biosensors.
Such meters are well known and normally contain a means for converting the reflectance reading to an analyte concentration value. Such a means may be, for example, a comparator means for comparing the signal generated by the reflectance photometer with a table to determine the analyte concentration related to a specific signal. It may also be software which makes use of an algorithm to convert such a signal to such a value. Such meters also normally include a display device for displaying the value.
When used to assay for an analyte in blood, many well known teststrips require that the indicator portion of the strip be dosed with blood and then washed, blotted or wiped free of excess blood after a specific reaction time. The blotting or wiping requirement is sometimes required to expose the reagent site for either visual or meter interrogation. Wiping is also normally required to avoid fouling a meter with excess blood whenever a teststrip is to be inserted into a meter for optical interrogation.
Although great commercial success has been achieved based on teststrips which must be wiped or blotted, there is room for improvement to achieve both more accurate assays and greater customer acceptance. The blotting or wiping step is believed to be a source of error in teststrip assays because some users wipe too lightly, leaving a layer of blood covering the reaction site. This layer of blood can mask or confuse the color change in the analyte, resulting in incorrect readings. Likewise, other users wipe the teststrip with too great a pressure, removing not only excess blood but also some of the reagent material, also resulting in an incorrect determination. The timing of the wiping step is also sometimes important. Incorrect readings can result if the wiping is accomplished too soon or too late.
The wiping step is also seen as an inconvenience by teststrip users who are required to provide the wiping material, such as cotton, and also to dispose of used wiping materials.
Much effort has been invested in forming a teststrip which does not have to be wiped and which will hold a useful amount of specimen at an operative site. One prior art design involves a teststrip having located, near its reagent site, a wick which directs excess specimen into a sink, thus avoiding the need to wipe away the excess. However, most such wicks draw away too much specimen, leaving too little at the reaction site. Many other efforts have been made to hold an operative amount of specimen at the reaction site while at the same time removing excess specimen without wiping. One example of such prior art designs uses soluble barrier layers to temporarily separate the reaction site from the wick; however, such designs present large manufacturing and materials selection problems.
A commercially available meter / teststrip system which attempts to solve the wiping problem requires that the teststrip be positioned in the meter and carefully aligned before a drop of blood is touched to a blood-receiving site on the strip. Some of the disadvantages of the wiping step have been avoided by this system; however, dosing of the teststrip in the meter by diabetics, who are sometimes unsteady and possessed of poor eyesight, is seen as a new disadvantage. Such persons also sometimes have trouble accurately placing a blood drop from a finger prick onto the small target area of the teststrip without getting blood on the surrounding meter parts. This excess blood can foul the mechanical and optical workings of the meter or lead to contamination in a clinical setting.
There exists a need for a teststrip which both hold a controlled amount of specimen at an operative site and avoids the requirement for wiping away excess sample and which also minimizes the possibility of fouling the meter with excess or badly applied sample.